<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>Document</title>
    <style>
      body {
        height: 2000px;
      }

      .back {
        position: fixed;
        width: 50px;
        height: 50px;
        background-color: pink;
        right: 30px;
        bottom: 100px;
        display: none;
      }

      .container {
        width: 900px;
        height: 500px;
        background-color: skyblue;
        margin: 400px auto;
      }
    </style>
    <script src="js/jquery.min.js"></script>
  </head>

  <body>
    <div class="back">返回顶部</div>
    <div class="container"></div>
    <script>
      // 写的少 做的多
      $(function () {
        var containerTop = $(".container").offset().top;
        // console.log(containerTop);
        $(window).scroll(function () {
          //   console.log($(document).scrollTop());
          if ($(document).scrollTop() >= containerTop) {
            $(".back").fadeIn(2000);
          } else {
            $(".back").fadeOut(2000);
          }
        });

        $(".back").click(function () {
          $("body,html").stop().animate({
            scrollTop: 0
          });
        });
      });
    </script>
  </body>
</html>
